Windows/Mac/Linux: After the release of Firefox 4, Firefox moved to an accelerated, Chrome-like channel system, starting with the bleeding-edge Aurora version of Firefox 5. Today they've released the Beta version, which will let you take advantage of its performance increases in a more stable environment.

Mozilla is pleased to release a new Firefox Beta for download and testing.
Mozilla recently introduced the Firefox Aurora development channel to support more frequent releases. The shift to a rapid release development cycle delivers cutting edge Firefox features, performance enhancements, security updates and stability improvements to users faster.

The main point of the article is - they released an option to switch between several release channels. Earlier you would have to download a new installer every time you wanted to switch versions, but now you can switch right from the browser.